drm/xe: Move XE_PTE_FLAG_READ_ONLY to xe_vm_types.h

XE_PTE_FLAG_READ_ONLY is specific to struct xe_vma, move it from xe_bo.h
to xe_vm_types.h to reflect that.

Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>